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Nobel Lecture

Nobel Lecture, 8 December 1981

Some Effects of Disconnecting the Cerebral Hemispheres

Introduction: Classic View of Cerebral
Dominance
To start by looking back a little, recall that even a small brain
lesion, if critically located in the left or language hemisphere,
may selectively destroy a person's ability to read, while at the
same time sparing speech and the ability to converse. The printed
page continues to be seen, but the words have lost their meaning.
This condition typically follows from focal damage to the angular
gyrus in the left hemisphere. It also results from lesions
interrupting the neural input to this left angular gyrus from the
visual or calcarine cortical areas (1,
2). It is natural to conclude in such
cases that the left hemisphere is responsible for reading while
the undamaged right hemisphere, in contrast, must be 'wordblind'
or incapable of seeing meaning in the printed word.

The same applies with respect to the
capacity to comprehend spoken words. Focal lesions within
Wernicke's area near the base of the left temporal lobe or,
again, lesions that disconnect this area from its input arriving
from the auditory receiving centers of the cortex have been shown
to regularly abolish the capacity to understand spoken language
(2). Speech continues to be heard but
the meaning is lost. Again, such cases seem to tell us that word
comprehension is confined to the left hemisphere and that the
spared right hemisphere must be word-deaf, as well as
word-blind.

The accumulation of many observations of
this kind where left, but not right, focal damage destroys the
comprehension, as well as the expression, of language helped to
give rise over the years to the so-called classic view in
neurology of a dominant or major, left, language hemisphere and a
subordinate, or minor, nonlanguage hemisphere. The minor
hemisphere in addition to being unable to talk, and unable to
write, and word-deaf and word-blind, was inferred by
extrapolation to be typically lacking also in the higher
cognitive faculties associated with language and symbolic
processing.

This classic view of cerebral dominance was
further reinforced by parallel findings on apraxia in which
disorders of learned volitional movement were reported to follow
predominantly lesions on the left side. The left hemisphere
accordingly came to be regarded as being also the leading motor
executive for the direction and control of higher volitional
movements and the major repository for the cerebral engrams of
motor learning (3, 4). Evidence for left dominance extended further
to calculation and arithmetic reasoning (5). Thus, with few exceptions, the bulk of the
collected lesion evidence up through the 1950s into the early
'60s converged to support the picture of a leading, more highly
evolved and intellectual left hemisphere and a relatively
retarded right hemisphere that by contrast, in the typical
righthander brain, is not only mute and agraphic but also
dyslexic, word-deaf and apraxic, and lacking generally in higher
cognitive function.

Contrasting evidence from
commissurotomy
It thus came as a considerable surprise in the early 1960s when
tests on commissurotomy or 'split-brain' patients seemed to
indicate the presence in the right, so-called 'minor' hemisphere
of a considerable capacity for cognitive understanding and the
comprehension of language, both written and spoken. These were
patients of the neurosurgeons Joseph Bogen and his chief, Phillip
Vogel of the White Memorial Medical Center in Los Angeles. The
patients had undergone a midline surgical section of the corpus
callosum and other forebrain commissures in a last resort effort
to control severe, intractable epilepsy. The operation severed
all neural cross connections for direct communication between the
two hemispheres. From experience with this operation in human
patients (6) and from nearly 10 years of
split-brain animal studies (7), it could
be predicted that the effect would not be seriously
incapacitating as far as ordinary daily activities were
concerned, and this proved to be the case. Given six months to a
year for recovery, and in the absence of other major brain
pathology, a person with complete section of the forebrain
commissures would go undetected as a rule in a casual first
meeting or conversation or even through an entire routine medical
exam.

Our early studies with Michael Gazzaniga
(8, 9, 10) on these patients seemed to show from the
start that the disconnected right hemisphere was by no means
word-deaf as anticipated, nor either word-blind. Lateralized
testing for linguistic abilities showed the right hemisphere to
be largely mute and agraphic, but nevertheless able to
comprehend, at a moderately high level, words spoken aloud by the
examiner. The disconnected right hemisphere also was able to read
printed words flashed to the left visual field - as demonstrated
manually in each case by selective retrieval or by pointing to
corresponding objects or pictures in a choice array. The
commissurotomy patients were also able with the right hemisphere
to choose correct written or spoken words to match presented
objects or pictures and to go correctly from spoken to printed
words and vice versa. Correct tactual retrieval by the right
hemisphere was achieved for objects not directly named but only
described with complex spoken phrases like "a measuring
instrument", "container for liquids", etc. With the disconnected
right hemisphere, these patients could also spell three and four
letter words with cutout letters and could read such words
presented tactually. These semantic capabilities of the right
hemisphere have more recently been affirmed and extended in a
comprehensive series of experiments by Zaidel (11) using his improved scleral lens technique
that allows prolonged viewing. So strong was contemporary
neurological doctrine to the contrary in the early sixties that
Dr. Bogen felt obliged in good conscience to withdraw his name
from our initial papers on language.

Our own conviction that the answers on
these language tests had to be coming from the right and not from
the left half of the brain was based on lateralized testing
procedures in which the speaking left hemisphere could be shown,
by follow-up verbal questions, to have remained incognisant or
quite unaware of the answers and performances being ascribed to
the right hemisphere. Each disconnected hemisphere behaved as if
it were not conscious of cognitive events in the partner
hemisphere - just as had been the case in our split-brain animal
studies of the 1950s started by Ronald Myers (12) at the University of Chicago. Each brain
half, in other words, appeared to have its own, largely separate,
cognitive domain with its own private perceptual, learning and
memory experiences, all of which were seemingly oblivious of
corresponding events in the other hemisphere. Although the basic
hemisphere deconnection syndrome in man (10) proved to be essentially similar to that
worked out earlier in cats and monkeys, its manifestation was
much more dramatic in the human subjects. The speaking hemisphere
in these patients could tell us directly in its own words that it
knew nothing of the inner experience involved in test
performances correctly carried out by the mute partner
hemisphere. Lateralization of brain functions could be inferred,
not only from the deficiency or absence of function on one side
but also from its concurrent presence on the other.

Right hemisphere language
controversy
The unexpected language capacities found in the right hemisphere
after commissurotomy posed some controversial issues the answers
to which are still not entirely resolved. Very simply, the
problem raised is the following: Why is it that the right
hemisphere is able to do things following commissurotomy, such as
reading, that it fails to do in the presence of focal damage in
the left hemisphere? It has been suggested in answer (13, 14, 15) that the commissurotomy evidence may be
misleading because of an atypical bilateral spread of language
into the right hemisphere correlated with the long-term epilepsy
and associated pathology. A further criticism has invoked
individual variation in view of the small patient group
involved.

We have favored another interpretation
which suggests conversely that it is the unilateral lesion
evidence that has been misleading. The reasoning here says that
left lesions in the presence of the commissures act to prevent
the expression of latent function, actually present but
suppressed, within the undamaged right hemisphere (10). This interpretation assumes that the two
halves of the brain, when connected, work closely together as a
functional unit with the leading control being in one or the
other. When this unitary function is rendered defective by a
one-sided lesion, the resultant impaired function prevails with
respect to both hemispheres. That is, the two continue to operate
as an integral, though defective, functional unit. Only after the
intact right hemisphere is released from its integration with the
disruptive and suppressive influence of the damaged hemisphere,
as effected by commissurotomy, can its own residual function
become effective.

This interpretation found support also in
the limited hemispherectomy data available (16). The same reasoning has seemed to apply as
well to phenomena of unilateral neglect and apraxia neither of
which proved to be nearly so severe in lateralized tests after
commissurotomy as one might have expected from the lateral lesion
findings. Although the final word on these various issues is not
yet in, the foregoing interpretation has received considerable
support in subsequent commissurotomy studies which reveal the
presence in the disconnected right hemisphere of additional
superior cognitive capacities that can hardly be ascribed either
to an atypical bilateralization of language or, any longer, to
individual variation. There is reason to think that these other
faculties also had gone unrecognized because of complexities that
inevitably prevail in the presence of the commissures.

Right hemisphere
specialization
Earlier indications of right hemisphere specialization in the
lateral lesion data, such as in facial recognition, dressing,
making block designs, drawing threedimensional cubes, etc., had
been ascribed to asymmetry in the sensory and motor-executive
realms primarily rather than in higher central cognitive levels.
These right hemisphere functions were referred to as
'visuospatial', 'constructional', or 'praxic'. In keeping with
conventional conceptions of cerebral dominance, any higher
cognitive processing that might be involved in such activities
could be assumed to be contributed from the left hemisphere via
the commissures. Our own initial interpretations of these
activities did not depart substantially from the classic view
(17).

By 1967, however, the collected
observations on the commissurotomy subjects were being taken to
uphold the conclusion (18) that each of
the disconnected hemispheres, not only the left, has its own
higher gnostic functions. Each hemisphere in the lateralized
testing procedures appeared to be using its own percepts, mental
images, associations and ideas. As in the split-brain animal
studies, each could be shown to have its own learning processes
and its own separate chain of memories, all of course,
essentially inaccessible to conscious experience of the other
hemisphere.

Added evidence for involvement of the right
hemisphere in higher intellectual processing came from study of a
case of congenital absence of the corpus callosum with an
above-average verbal IQ and in whom speech was found to be
present in the right as well as the left hemisphere (19, 20). The
scholastic records of this college student with callosal agenesis
were fair to good for courses that involved language and verbal
facility, but contrastingly poor for subjects such as geometry
and geography that involved spatial and related nonverbal
faculties which we now commonly associate with the right
hemisphere. The extra language in the right hemisphere had
apparently been attained at the expense of the usual nonverbal
cognitive faculties that otherwise normally develop there.

More direct, controlled evidence for right
hemisphere superiority in tasks requiring higher cognitive
ability came from studies by Jerre Levy (21, 22) aimed
specifically at cognitive specialties of the right hemisphere.
She found that the mental capacity to make intermodal spatial
transformations from three-dimensional to unfolded,
two-dimensional forms was much better developed in the right
hemisphere. Also where items in the test series showed higher
scores by the left hemisphere there was a corresponding drop in
right hemisphere performance suggesting a left-right polarity in
cognitive abilities.

From these data, taken in conjunction with
available clues from the literature, Levy proposed that left and
right hemispheres are characterized by inbuilt, qualitatively
different and mutually antagonistic modes of cognitive
processing, the left being basically analytic and sequential, the
right spatial and synthetic. A rationale was added for the
evolution of cerebral asymmetry (23)
based on the functional advantages of having the two cognitive
modes develop in separate hemispheres in order to minimize mutual
interference.

In succeeding years thinking evolved
rapidly along these lines and became strengthened and refined
through a series of studies (24-31) in which it
proved possible to demonstrate further that the so-called
subordinate or minor hemisphere, which we had formerly supposed
to be illiterate and mentally retarded and thought by some
authorities to not even be conscious, was found to be in fact the
superior cerebral member when it came to performing certain kinds
of mental tasks. The right hemisphere specialities were all, of
course, nonverbal, nonmathematical and nonsequential in nature.
They were largely spatial and imagistic, of the kind where a
single picture or mental image is worth a thousand words.
Examples include reading faces, fitting designs into larger
matrices, judging whole circle size from a small arc,
discrimination and recall of nondescript shapes, making mental
spatial transformations, discriminating musical chords, sorting
block sizes and shapes into categories, perceiving wholes from a
collection of parts, and the intuitive perception and
apprehension of geometrical principles. The emphasis meantime
became shifted somewhat from that of an intrinsic antagonism and
mutual incompatibility of left and right processing to that of a
mutual and supportive complementarity.

In many cases the observed left-right
cognitive differences were rather subtle and qualitative in
nature, such that they would easily be obscured in lateral lesion
studies by individual variation and background pathology. Under
the conditions of commissurotomy where background factors are
equalized and where close left-right comparisons become possible
within the same subject working the same problem, even slight
lateral differences become significant. The same individual can
be observed to employ consistently one or the other of two
distinct forms of mental approach and strategy, much like two
different people, depending on whether the left or right
hemisphere is in use.

Further extensions
Further developments from other sources have advanced in many
directions through study of various normal, brain-damaged and
other select populations (32, 33), exploring correlations with handedness,
gender, occupational preferences and ability, special innate
talents, genetic variations like Turner's syndrome, congenital
dyslexia, endocrine pathology, autism, dreaming, hypnosis,
inverted writing - and others. In some cases the conclusions
along with the growing wave of semipopular extrapolations and
speculations concerning "leftbrain" vs. "right-brain" functions
call for a word of caution. The left-right dichotomy in cognitive
mode is an idea with which it is very easy to run wild.
Qualitative shifts in mental control may involve up-down,
front-back, or various other organizational changes as well as
left-right differences. Furthermore, in the normal state the two
hemispheres appear to work closely together as a unit, rather
than one being turned on while the other idles. Much yet remains
to be settled in all these matters. Even the main idea of
differential left and right cognitive modes is still under
challenge in some quarters in favor of the view that the right
hemisphere specialities are primarily praxic or 'manipulospatial'
in character and that higher cognition and self awareness are
associated mainly with language in the left hemisphere (34, 35).

Regardless of remaining uncertainties
concerning laterality, one beneficial outcome that appears to
hold up is an enhanced awareness, in education and elsewhere, of
the important role of nonverbal components and forms of
intellect. Another broadly relevant outcome, that derives from
evidence involving familial, mutational, sexual and other innate
variations, is a growing recognition of, and respect for the
inherent individuality in the structure of human intellect. The
more we learn, the more complex becomes the picture for
predictions regarding any one individual and the more it seems to
reinforce the conclusion that the kind of unique individuality in
our brain networks makes that of fingerprints or facial features
appear gross and simple by comparison. The need for educational
tests and policy measures to selectively indentify, accommodate,
and maximize the differentially specialized forms of individual
intellectual potential becomes increasingly evident.

Self consciousness and social
awareness
Earlier contentions that the right hemisphere is not even
conscious largely gave way by the mid seventies to an
intermediate position conceding that the mute hemisphere may be
conscious at some lower elemental levels, but claiming that it
lacks the higher, reflective, self-conscious kind of inner
awareness that is special to the human mind and is needed, so it
is said, to qualify the right conscious system as a "self' or
"person" (36, 37). Self awareness in particular is reported,
on the basis of mirror tests mainly, to be a predominantly human
attribute and is rated by developmental as well as by
evolutionary standards to be a highly advanced phase of conscious
awareness.

Accordingly we undertook to test the right
hemisphere more specifically for the presence of self recognition
and related forms of self and social awareness. With perception
of pictorial stimuli confined to one hemisphere by the scleral
contact lens occluder developed by Eran Zaidel (38), the subject merely had to point to select
items in a multiple choice array in answer to various kinds of
leading questions regarding his or her knowledge and feelings
concerning the content of the pictures. Subject's responses
included also differential emotional expressions, thumbs-up,
thumbs-down evaluations, exclamations, replies to 20-question
type prompting and spontaneous remarks relevant to the emotional
aspects of affect-laden stimuli.

The results (39)
revealed that the disconnected right hemisphere readily
recognizes and identities him or herself among a choice array of
portrait photos, and in doing so, generates appropriate emotional
reactions and displays a good sense of humor requiring subtle
social evalulations. Similar findings were obtained for pictures
of the immediate family, relatives, acquaintances, pets, personal
belongings, familiar scenes and also political, historical and
religious figures, as well as television and screen
personalities. The relatively inaccessible inner world of the
nonspeaking hemisphere was thus found to be surprisingly well
developed. The general level of performance on these tests was in
good accord with that obtained from the left hemisphere of the
same subject or in free vision. Results to date suggest the
presence of a normal and well developed sense of self and
personal relations along with a surprising knowledgeability in
general.

Similar projective procedures were used to
explore for a sense of time in the right hemisphere and the
presence of concern for the future with thus far no evidence of
abnormal deficit. The nonvocal hemisphere appears to be quite
cognisant of the person's daily and weekly schedules, the
calendar, seasons, and important dates of the year. The right
hemisphere also makes appropriate discriminations that show
concern with regard to the thought of possible future accidents
and personal or family losses. The need for life, fire, and theft
insurance, for example, seems to be properly appreciated by the
extensively tested mute hemisphere of these patients.

Unlike other aspects of cognitive function,
emotions have never been readily confinable to one hemisphere.
Though generated by lateralized input, the emotional effects tend
to spread rapidly to involve both hemispheres, apparently through
crossed fiber systems in the undivided brain stem. In the above
tests for self consciousness and social awareness it was found
that even subtle shades of emotion or semantic connotations
generated in the right hemisphere could be quite helpful to the
left hemisphere in its efforts to guess the nature of a stimulus
known only to the right hemisphere. The results suggested that
this affective, connotational or semantic component could play an
extremely important role in cognitive processing generally.

The more structured and specific
informational components of cognitive processing were shown to be
separable from the emotional and connotational components. The
former remained confined within the hemisphere in which it was
generated, whereas the emotional overtones leaked across to
influence neural processing in the other hemisphere. The evidence
of this separability is in itself significant in regard to
questions of the organization of the neural mechanisms of
cognition. Also, since the affective component appears to be an
eminently conscious property, the fact that it crosses at lower
brainstem levels is of interest in reference to the structural
basis of consciousness. A major thrust in our current work is
aimed at determining more precisely what shades of emotional,
connotational or semantic content are able to cross through the
brainstem and how they affect cognitive processing on the other
side. In these studies we are using a new technique just
developed for lateralizing vision (40,
41). It allows prolonged viewing
without attachments to the eye.

Progress on mind-brain problem
In closing it remains to mention briefly that one of the more
important things to come out of the split-brain work, as an
indirect spin-off, is a revised concept of the nature of
consciousness and its fundamental relation to brain processing
(42, 43,
44). The key development here is a
switch from prior non-causal, parallelist views to a new causal,
or "interactionist" interpretation that ascribes to inner
experience an integral causal control role in brain function and
behavior. In effect, and without resorting to dualist views, the
mental forces and properties of the conscious mind are restored
to the brain of objective science from which they had long been
excluded on materialist-behaviorist principles.

Acceptance of the revised "causal view" and
the reasoning involved, now becoming widespread, carries
important implications for science and for scientific views of
man and nature. Cognitive introspective psychology and related
cognitive science can no longer be ignored experimentally, or
written off as "a science of epiphenomena", nor either as
something that must, in principle, reduce eventually to
neurophysiology. The events of inner experience, as emergent
properties of brain processes, become themselves explanatory
causal constructs in their own right, interacting at their own
level with their own laws and dynamics. The whole world of inner
experience (the world of the humanities) long rejected by 20th
century scientific materialism, thus becomes recognized and
included within the domain of science.

Basic revisions in concepts of causality
are involved in which the whole, besides being "different from
and greater than the sum of the parts", also causally determines
the fate of the parts, without interfering with the physical or
chemical laws for the subentities at their own level. It follows
that physical science no longer perceives the world to be
reducible to quantum mechanics or to any other unifying ultra
element or field force. The qualitative, holistic properties at
all different levels become causally real in their own form and
have to be included in the causal account. Quantum theory on
these terms no longer replaces or subsumes classical mechanics
but rather just supplements or complements.

The results add up to a fundamental change
in what science has long stood for throughout the
materialist-behaviorist era (45). The
former scope of science, its limitations, world perspectives,
views of human nature, and its societal role as an intellectual,
cultural and moral force all undergo profound change. Where there
used to be a chasm and irreconcilable conflict between the
scientific and the traditional humanistic views of man and the
world (46, 47), we now perceive a continuum. A unifying
new interpretative framework emerges (48) with far reaching impact not only for
science but for those ultimate value-belief guidelines by which
mankind has tried to live and find meaning.

Acknowledgments
Our split-brain studies could hardly have succeeded without the
contributions of a long line of very able graduate students and
postdoctoral associates. I am particularly grateful to Ronald
Myers who started the animal work in his doctoral research at the
University of Chicago; Michael Gazzaniga, first to work with the
human subjects and Jerre Levy, first to demonstrate superior
cognitive processing in the right hemisphere. All contributed
immensely to these respective innovations as did others to more
specific aspects of the program. We are deeply indebted to Drs.
Joseph Bogen and Phillip Vogel for generously making their
patients available for study, and to the patients themselves
without whose long and willing cooperation the human work would
not have been possible.

Our work has been dependent for funding
since the late 1950s on successive federal grants conferred
mainly by the National Institutes of Mental Health. My chair at
Caltech was made possible and has been sustained throughout by
the F. P. Hixon Fund of the California Institute of Technology
donated to bring to the Institute research bearing on "the 'why'
of human behavior."

For research assistance in the human
studies we owe much to the dedicated efforts of Dahlia Zaidel
over a 9 year period beginning in 1967, and to those also of Lois
MacBird in both the animal, and more recently, the human work
extending over a 25 year period to the present. Our research
progress has been dependent in no small measure on the consistent
support received on all sides at Caltech. My own efforts could
not have prospered without the constant help and understanding of
Norma, my wife, whose competence and willingness in handling
matters of our home and family has freed me over the years to
give added time to problems of the laboratory.